In the quantum cryptographic communication system, loading each photon with information enables the detection of an unauthorized listener by the quantum mechanical principle. However, if the same information is loaded on two or more photons, the unauthorized listener may utilize a part of these photons and the presence of the unauthorized listener may not be able to be detected. In this way, ideally, a pulse that contains only one photon at maximum must be used. For this kind of pulse, it is popularly practiced to attenuate the light beam from the laser beam source by an attenuator in such a manner that the mean number μ of photons per pulse becomes about 0.1. By doing this, the probability to contain two or more photons in a pulse can be reduced to 1/100, but the probability to contain one photon in the pulse is also reduced to about 0.1. That is, in case of μ=0.1, transmission is actually carried out only about once per 10 times.
As one example of conventional techniques for improving this kind of process, description will be made on the case stated in the “Key Distribution system and method using Quantum Cryptography” of Japanese Unexamined Patent Publication No. 8-505019 (1996) referring to FIG. 11, which corresponds to FIG. 5 in the embodiment of the said invention. In FIG. 11, numeral 7 denotes a laser that generates pumping light 8 for pumping the nonlinear optical medium 9. In the nonlinear optical crystal 9, a parametric fluorescence pair that causes one photon of the pumping light to stochastically generate two photons is generated. One photon of these (in this case, called the “idler photon 5”) is detected by an optical detector and a gate controller 38, and when detected, the gate device 4 is opened to enable the other photon (called the “signal photon 6”) to pass. In these conventional techniques, as described in the embodiment of the “Key Distribution system and method using Quantum Cryptography” of Japanese Unexamined Patent Publication No. 8-505019 (1996), for the optical detector, a photo-multiplier tube or a semiconductor avalanche photodiode (hereinafter called “AQ-APD”) used under active quenching control was used.
However, if the detector used in the conventional technique is used as a photon detector as it is, when a plurality of photons were incident in a response time of the detector, the detector was unable to detect that a plurality of photons were incident. For example, in AQ-APD, since the amplitude of the pulse becomes constant irrespective of the number of incident photons, it was able to detect that photons were incident but it was unable to obtain the information on the number of incident photons. In addition, when the photo-multiplier tube is used, the quantum efficiency η is as low as about 20% at maximum, and the detection efficiency of two photons is extremely low (4%) because the efficiency is equal to the square of η, and it was unable to detect the incidence of two or more photons.
In the conventional technique, because the use of these detectors was premised, there was no means for judging the number of incident photons.
Consequently, even when a plurality of photons are generated sequentially from a parametric fluorescent pairs within a range of the response time of the detector, the detector opens the gate, causing a problem in that a two-photon-state emerges.
However, on the other hand, it was impossible to generate the “two-photon-state” in which two photons sequentially exist within a range of the response time of the detector.
As described above, because in the conventional technique, the detector was unable to detect the number of incident photons and had no means for judging the number of incident photons, the detector had a drawback in that even if photon pairs were generated sequentially within the response time of the photon detector, it opened the gate and a state in which two or more photons were contained within the response time of the detector was emitted.
Furthermore, in the conventional method, it was unable to accurately generate a plurality of photons in one pulse.
It was also unable to control the timing of photon generation in the pulse.
The present invention was achieved to solve these problems, and it is an object of the present invention to provide a device that can suppress uncorrelated two or more photons from being contained in the response time of the detector, to generate an accurate of correlated photons in one pulse, and to control the timing of photon generation in a pulse.